I'm on T mobile, why would that make a difference?
Network compatibility was already brought up, but here's another reason. Let's say you have a phone in mind but debating on which carrier. One could have a close tower giving you great reception, but the other with a tower further away and making your phone work harder to pull in the signal. All things being equal in this example, the phone would have worse battery life on the weaker network. You'd be surprised how much network signal strength can impact battery life, and that's just one reason why your initial question is hard to answer.
If the absolute battery life is that important, then I'd consider a couple other options. Get something with a removable battery (the LG G4 has a good reputation on the stock battery and it's removable), then replace it with a larger aftermarket battery. With the G4, I've seen batteries up to 10,000mAh. It's not without some trade offs in weight and size, but may be worth it.
Another option would be get a phone case with a built in battery. These cases are like external power packs that plug in to your USB port to charge/power your phone but are built into the case itself.
Either option may be what you need to go a full two days depending on your situation.